The invention is a process for recovering hydrocarbons from an underground hydrocarbon formation. More particularly, the invention relates to a method of employed in situ combustion wherein the combustion front is quenched with water and reignited in order to shorten the flood life and improve the economics of fireflooding.
It is well recognized that primary hydrocarbon recovery techniques may recover only a portion of the petroleum in the formation. Thus, numerous secondary and tertiary recovery techniques have been suggested and employed to increase the recovery of hydrocarbons from the formations holding them in place. Thermal recovery techniques have proven to be effective in increasing the amount of oil recovered from the formation. Waterflooding and steamflooding have proven to be the most successful oil recovery techniques yet employed in commercial practice. Successes have also been achieved with in situ combustion processes.
An in situ combustion process requires the injection of sufficient oxygen-containing gas such as air to support and sustain combustion of the hydrocarbons in the reservoir. When the flow of the oxygen-containing gas in the reservoir is large enough, combustion will occur, either spontaneously or from an external heat source such as a downhole heater. A portion of the oil is burned as fuel at the high temperature front which proceeds slowly through the reservoir, breaking down the oil into various components, vaporizing and pushing the oil components ahead of the burning regions through the reservoir to the production wells.
Several methods have been suggested to improve in situ combustion drives. The most effective of these has been the method of wet combustion. In this case, a combustion drive is converted into wet combustion by the co-injection or alternate injection of water along with the oxygen-containing gas for combustion. A portion of the water that is injected flashes ahead of the combustion front to form a larger steam plateau which helps provide for greater displacement and oil recovery than a dry combustion process. Wet combustion offers the advantages of higher oil recovery, higher combustion front velocity, and lower fuel and air requirements than dry combustion.
Several combustion methods have been disclosed in which an in situ combustion process has been quenched as a floodout stage by the injection of water near the end of combustion. These processes do not disclose the quenching of a combustion drive and refrain from such a step prior to reaching the end of the combustion phase of a method. No mention is made of the reignition of a quenched combustion drive during, or after combustion.
U.S. Pat. Nos. 3,991,828 and 4,059,152 disclose methods for recovering viscous crude by first injecting superheated steam, initiating a combustion drive followed by wet combustion, and concluding with the injection of water when the wet combustion front reaches the production wells. U.S. Pat. No. 4,495,994 discloses a process with the injection of high quality steam, followed by the injection of steam of a decreasing quality, followed by wet in situ combustion, and finally, 100% water injection without air for combustion floodout near the production wells. U.S. Pat. No. 3,150,715 also discloses a wet in situ combustion process wherein injection of air is discontinued while water injection is continued, but only once the combustion front has reached production wells.
Although it is generally considered desirable to operate a wet combustion drive at a high water to air ratio, it is considered undesirable to quench the combustion front prior to reaching the area of the production wells. The literature constantly warns against injecting an excesive quantity of water and extinguishing the combustion front. This may be because of ignition problems which have been experienced in some combustion projects in the field.